1. Field of the Invention
The present invention relates to a linear guide apparatus having a slider movable slidingly with respect to a guide rail, and in particular, to an improved linear guide apparatus in which it is made possible to control the frictional resistance between the guide rail and the slider.
2. Prior Art
Generally, a linear guide apparatus includes a guide rail, a slider movable along and guided by the guide rail, and many balls interposed between the guide rail and the slider. The slider moves slidingly and smoothly along the guide rail due to the rolling movement of the balls. However, since the sliding resistance of the slider is very small, it is sometimes difficult to stop the slider at a desired position. For this reason, braking means are provided between the slider and the guide rail.
Such linear guide apparatus are disclosed, for example, in Japanese Utility Model Laid-Open Publication No. 58-6010 (first prior art), and Japanese Patent Laid-Open Publication No. 61-11611 (second prior art) filed by the same applicant as the present application.
In the first prior art, a sliding bearing made of low friction material and loaded with a spring is interposed between a guide rail and a slider, so as to increase the frictional resistance between the guide rail and the slider to some degree and to provide a damping force at the time of stopping after the slider travels a predetermined distance.
On the other hand, in the second prior art, a friction member loaded with a spring member is provided on a surface of the slider opposing the guide rail. The friction member can be moved away from or brought into contact with the guide rail by means of a screw. Thus, in mounting the slider on the guide rail, the friction member is separated from the guide rail by the screw to facilitate assembly. After assembly, the screw member is removed so as to bring the friction member into contact with the guide rail, to press against the guide rail, so that a damping force is produced at the time of stopping the slider after it has travelled a predetermined distance.
However, in the first and second prior art, since the damping force at the time of stopping the slider is provided by the pressing force of the spring, there are drawbacks in that the damping force is fixed to a predetermined value and it is impossible to change the braking characteristics. Also, the pressing force of the spring is decreased due to deterioration with age and, as such, it become difficult to stop the slider at a fixed position. Furthermore, since there are variations in spring load, it is difficult to maintain the damping force at a predetermined value. Also, an unsolved problem is present in that the spring cannot be used positively as a brake in stopping the slider after it has travelled to a predetermined position. As a result, when the linear guide apparatus of the prior art is used in machine tools for performing heavy cutting, if the braking force is set to match the working condition a large braking force will be exerted even in such conditions in which the table returns to the original position without performing the work, or when the table makes access to the workpiece. Thus, a large loss of power will be caused when the table is moved rapidly, and also, since the load applied to the feed screw is large at the time of rapid feeding, the life of the machine will be short.